1. Field of the Invention
The present invention concerns generally a power amplifier, and more particularly it pertains to an amplifier of the power supply changeover type which is used in, for example, high fidelity reproduction of signals, and is operative so that the power supply voltage which is supplied to a power amplifier is varied in accordance with the level of the input signal which is to be amplified thereby, to improve the power efficiency.
2. Description of the Prior Art
In such conventional amplifier as shown in FIG. 1 which is intended for such high efficient reproduction of signals in audio systems, a signal applied across input terminals 1a and 1b is amplified in a power amplifying circuit 2 before being supplied to a loudspeaker serving as a load 4 connected between output terminals 3a and 3b. In case the signal which is amplified by the power amplifying circuit 2 is of a low level, i.e. small in amplitude, switches 5a and 5b are rendered "off" so that output voltages +V.sub.1 and -V.sub.3 of power supplied 6a and 6b are supplied to said power amplifying circuit 2 via diodes 7a and 7b, respectively. On the other hand, in case the signal is of a high level, i.e. a large amplitude, the switches 5a and 5b are rendered "on", so that a sum output voltage +(V.sub.1 +V.sub.2) of the power supplies 6a and 8a, and a sum output -(V.sub.3 +V.sub.4) of the power supplies 6b and 8b are supplied to the power amplifying circuit 2 via the switches 5a and 5b, respectively.
In such amplifier, the power supply voltages indicated by solid lines b.sub.1 and b.sub.2 are varied in accordance with the level indicated by a solid line a of the output signal of the power amplifying circuit 2 as shown in FIG. 2. Whereby, that power loss in the power amplifying circuit 2 corresponding to the hatched portion indicated in FIG. 2 is not developed, i.e. power efficiency is improved. Along therewith, the signal of a high level is outputted after being sufficiently amplified, without being clipped owing to a shortage of the power supply voltage.
It is to be noted here that the musical signals which are dealt with by such amplifier as mentioned above mostly are comprised of signals of low levels, or that the ratio of length of time which is occupied by only-seldom-appearing large amplitude signals in musical signals is very small, especially so in classical music. For this reason, in an amplifier of this type, it becomes necessary for materializing an amplifier as a whole having a compact size and a light weight to minimize the calorific power in its state that only the low voltage power supplies, i.e. power supplies 6a and 6b in FIG. 1, are used.
In FIG. 3 is shown a diagrammatic sectional view of a power transformer for power supplies 6a, 6b and 8a, 8b for the conventional amplifier shown in FIG. 1. This power transformer is comprised of a core 10, a primary coil 11 wound around this core 10, a first secondary coil 12 for the power supplies 6a and 6b, and a second secondary coil 13 for the power supplies 8a and 8b. In such power transformer, the second secondary coil 13 is intended for supplying a power only when a signal of a high level is amplified. Accordingly, the ratio of power supplying time through this winding 13 is very small as compared with the remaining windings. Nevertheless, during the period in which a power is being supplied through the winding 13, there also arises the necessity for supplying a large power. Accordingly, there is the need that this second secondary coil 13 possess a sufficiently large capacity, too. Further, there is required a smoothing capacitor of a large capacity, although not shown here, which is intended to smooth the rectified output of said second secondary coil 13.
On the other hand, as for the first secondary coil 12, there is needed a large size winding which generates as little heat as possible from the foregoing reasons, i.e. which has little copper loss, i.e. having a winding of a large diameter wire and having small resistance. However, in view of the fact that the abovesaid second secondary coil 13 occupies a large space, i.e. needing a large space factor, it will be understood that, in case it is intended to make the transformer compact in size, the first secondary coil 12 will be unable to secure such a sufficient space factor as it needs, and that accordingly the amount of its heat generation will become great. As discussed above, conventional amplifiers have the drawback that it has been difficult to materialize a power transformer having a compact size and a light weight so as to minimize a power amplifier in size and weight as a whole.